/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Preconditions;

/**
 * Implementation of {@link ImmutableSet} with exactly one element.
 * 
 * @author Kevin Bourrillion
 * @author Nick Kralevich
 */
@GwtCompatible(serializable = true)
@SuppressWarnings("serial")
// uses writeReplace(), not default serialization
final class SingletonImmutableSet<E> extends ImmutableSet<E> {
    final transient E element;

    // Non-volatile because:
    // - Integer is immutable and thus thread-safe;
    // - no problems if one thread overwrites the cachedHashCode from another.
    private transient Integer cachedHashCode;

    SingletonImmutableSet(E element) {
        this.element = Preconditions.checkNotNull(element);
    }

    SingletonImmutableSet(E element, int hashCode) {
        // Guaranteed to be non-null by the presence of the pre-computed hash
        // code.
        this.element = element;
        cachedHashCode = hashCode;
    }

    public int size() {
        return 1;
    }

    @Override
    public boolean isEmpty() {
        return false;
    }

    @Override
    public boolean contains(Object target) {
        return element.equals(target);
    }

    @Override
    public UnmodifiableIterator<E> iterator() {
        return Iterators.singletonIterator(element);
    }

    @Override
    public Object[] toArray() {
        return new Object[] { element };
    }

    @Override
    public <T> T[] toArray(T[] array) {
        if (array.length == 0) {
            array = ObjectArrays.newArray(array, 1);
        } else if (array.length > 1) {
            array[1] = null;
        }
        // Writes will produce ArrayStoreException when the toArray() doc
        // requires.
        Object[] objectArray = array;
        objectArray[0] = element;
        return array;
    }

    @Override
    public boolean equals(@Nullable Object object) {
        if (object == this) {
            return true;
        }
        if (object instanceof Set) {
            Set<?> that = (Set<?>) object;
            return that.size() == 1 && element.equals(that.iterator().next());
        }
        return false;
    }

    @Override
    public final int hashCode() {
        Integer code = cachedHashCode;
        if (code == null) {
            return cachedHashCode = element.hashCode();
        }
        return code;
    }

    @Override
    boolean isHashCodeFast() {
        return false;
    }

    @Override
    public String toString() {
        String elementToString = element.toString();
        return new StringBuilder(elementToString.length() + 2).append('[')
                .append(elementToString).append(']').toString();
    }
}
